Sewer Pump Station underperforming at Start Up

[djcbgn]

Hi,
I was asked to look at a new sewer pump station that is not performing as intended. The pumps are S&L suction lift pumps. This is a two pump series arrangement (attached) There is approximately 10,600 feet of 6inch pvc (sdr18) force main and about 19 feet of 4 inch suction pipe(20 max lift per S&L). The high point is at the pipe end (discharge manhole location).

Static Head= 177 feet (99'2-815')
Design Flow = 250 gpm
TDH (design) = 236’
Approx. Pump CL elevation 834. Pressure gauge at this elevation

During initial startup the contractor filled the pipe but during subsequent pump start the pump would run about 190 gpm at 90 psi and then stop. It did not seem like they hit dead head, the impellers were spinning according to the contractor but no there was no flow. When the pumps were off the static pressure was at 70 psi, which is correct. The contractor exercised the isolation valves and air release valves to make sure they were open and tried again with the same results. I estimated the dead head pressure on the discharge main to be around 106 psi.

The contractor then ran the pumps through the 4 inch emergency bypass with the valve throttled some and got about 275 gpm at 91 psi. The pump ran normal is this case. This flow and pressure is close to falling on the pump curve. Next they again ran the pumps through the bypass but throttled the valve until the pump dead headed at 106 psi.

I included my calculations with the system head curve and pump curve plotted. I am wondering if this could be air trapped in the line. There are 6 air release valve locations on the force main . There is an aerial stream crossing just downstream of the pump station and there was not an air valve at the pump station (or just outside of the pump station/see attached). If there is air constricting flow wouldn’t this show as increased pressure? I am unsure why the pumps stop pumping at 91 psi discharge pressure when dead head pressure is about 106 psi. I could add air release valve at the pump station, but wanted to see if anyone had any ideas to trying to isolate the problem areas.

Please let me know what you think. Thank for your input.

 

[jhnblgr]

What reducer was used 4 to 6 connection? How is the bypass plumbed? What amperage is the pump pulling? Is there a VFD?

 

[djcbgn]

An eccentric 4x6 ductile fitting.
Dont have the amperage. I will see if I can get this.
No VFDs
See attached for pump station site plan and bypass detail. I said 4 inch bypass earlier, but its a 6 inch.

I attached the pump station site plan and bypass detail.

The calcs I attached on the original post were the ones I did to compare to the calcs the original engineer prepared. My design point was similar.
Thanks.

 

[bimr]

Another uphill force main with problems!

Your operating pumping rate of 190 gpm doesn't have high enough velocity to push the air out. The trapped air results in a higher than anticipated head loss.

Not sure how you plan to operate. If the pump is operated intermittently, you need a velocity of at least 3.5 ft/sec to resuspend the solids that settle out when the pump starts.

Air valves on wastewater piping should be used to let the air out on project startup and then be closed. Otherwise, the solids will cause leakage.

 

[djcbgn]

I am considering getting a portable pump to pump through the bypass connection at a higher flow rate to try an move the air.

The design operating point was closer to 2.8 fps and about 3.2 fps with two sets running, but of course it never got there.

 

[bimr]

If you look at the horsepower calculator on this site:

Pump Power Calculator

Calculate pumps hydraulic and shaft power.

www.engineeringtoolbox.com

If you are using a standard no-clog centrifugal pump on this application, you pump motor is likely undersized as the pump efficiency may be as low as 40%.

 

[djcbgn]

If you look at the horsepower calculator on this site:

Pump Power Calculator

Calculate pumps hydraulic and shaft power.

www.engineeringtoolbox.com

If you are using a standard no-clog centrifugal pump on this application, you pump motor is likely undersized as the pump efficiency may be as low as 40%.

Thanks for your input. The pump curve is attached. These are 2-pumps w/20hp motor in series. I am not seeing where the motor would be a problem. I dont understand why the pumps stop running at about 91 psi discharge pressure when shut off head on should be around 106 psi. When they ran the pumps through the bypass w/valve throttled the flow/pressure were close to where they should be. I feel there is something in the system that is creating headloss, maybe air pocket(s). But I am not had this situation before so I am not sure.

 

[bimr]

I thought the pumps were in parallel. I will look at it again.

 

[fel3]

I thought the pumps were in parallel. I will look at it again.

It looks like Pumps 1 and 2 are in series, Pumps 3 and 4 are in series, and the two pump sets are in parallel. I have never seen this type of setup before (all of my small wastewater lift stations used submersible pumps). I haven't examined the calcs or spent much time looking at the drawings, but I did notice that the Pumps Off level requires a suction lift of approximately 19 feet static plus inlet and suction pipe losses. I wonder if this is part of the problem. I also don't like running pumps that far to the left on the curve.

 

[djcbgn]

It looks like Pumps 1 and 2 are in series, Pumps 3 and 4 are in series, and the two pump sets are in parallel. I have never seen this type of setup before (all of my small wastewater lift stations used submersible pumps). I haven't examined the calcs or spent much time looking at the drawings, but I did notice that the Pumps Off level requires a suction lift of approximately 19 feet static plus inlet and suction pipe losses. I wonder if this is part of the problem. I also don't like running pumps that far to the left on the curve.

Yes thats correct. This type of pump was a standard in this particular City, but normally pump stations are submersible usually with Flygt pumps. I do a lot of developer pump stations and the sites with high static head tend to have pumps that run far to the left. High head and not a lot of flow unless its a very large development, like 500 homes or more. For me its very common to see this and in some cases I need to go to a larger force main and increase the flow rate to move the operating point to the right. The max static head is 20 ft in this case, I confirmed this with the Smith and Loveless. The pump technician checked the vacuum priming system during startup with no problems.